A lithium-ion secondary battery has been mainly used for portable electronic apparatuses as a lightweight and high-energy density storage device. In recent years, a lithium-ion secondary battery has come to have higher power and has also been adopted for use in large vehicles such as a hybrid automobile and a hybrid rail car. As a tendency in developing a lithium-ion secondary battery, as shown in FIG. 5, the development is becoming bifurcated as follows: development toward a larger capacity and a higher energy density; and development toward a smaller capacity and a higher power density. In applications for a storage device, it is often the case that the peak power is high but only momentary. If, of the bifurcated storage devices described above, only larger capacity and higher energy density storage devices are used, energy more than necessary is mounted. On the other hand, if only the smaller capacity and higher power density storage devices are used, the configuration has power more than necessary. In this way, when any one of the two types of storage devices is adopted, the configuration has unnecessary energy or power, which leads to increases in weight, volume, and cost of storage systems. Therefore, there have been proposed cases in which a high power type storage device and a large capacity type storage device are connected via a bidirectional DC-DC converter.
For example, in a power supply system described in Patent Literature 1, a DC-DC converter is arranged only for an auxiliary secondary battery block. It is described that, through boosting an output voltage of the auxiliary secondary battery block to a level equivalent to that of a main secondary battery block using the DC-DC converter, the auxiliary secondary battery block and the main secondary battery block are operated in parallel, thereby charging from and discharging to a load. According to the invention described in Patent Literature 1, the main secondary battery block is always connected without the DC-DC converter, and the auxiliary secondary battery block is always connected via the DC-DC converter.
In Patent Literature 2, there is introduced, as an application for a electric vehicle, e.g., a hybrid automobile or an electric automobile, a configuration of a power supply system in which a second storage battery 2 connected via a bidirectional DC-DC converter so as to be in parallel to a first storage battery is boosted when used, and is controlled through opening/closing a relay.
In the configuration of Patent Literature 2, a relay is arranged so that two high voltage batteries may be solely connected and may be connected in parallel to each other. However, in such a configuration, a first storage battery is always connected without a DC-DC converter, and a second storage battery 2 is always connected via the DC-DC converter. When a storage battery connected via a DC-DC converter and a storage battery connected without the DC-DC converter are fixed in this way, the efficiency is lowered to narrow the range of applications depending on a load pattern. Further, when the lifetime of the storage batteries is taken into consideration, it is necessary to form a configuration taking into consideration the voltages and the capacities of the storage batteries.
On the other hand, when the configuration is formed so that a plurality of electric power supply paths can be obtained using a switch, various electric power paths are thought of, and thus, the design is required to be made taking into consideration safety. Although a secondary battery is rechargeable, in a lead-acid battery, a nickel metal hydride battery, or a NiCad battery in an overcharged state, water electrolysis occurs and the internal pressure increases and heat is generated. Meanwhile, in a lithium-ion secondary battery or an electric double-layer capacitor, an organic solvent is used in an electrolyte solution, and thus, there is a possibility that electrolyte solution decomposition reaction occurs.
In such a multiple-power supply system using a bidirectional DC-DC converter, high power storage devices can be connected in parallel through ON/OFF of a switch. However, depending on the pattern of power running and regeneration, energy is more efficiently given and received without a DC-DC converter when a storage device connected via the DC-DC converter is disconnected and a high power type storage device is mainly used. Meanwhile, when ON/OFF of the switch is performed, the electric power path is complicated and, in case of inconvenience, measures are required to be taken.